1. Field of Invention
This invention relates to lead-free and cadmium-free barium titanate based dielectric compositions, and more particularly to barium titanate based dielectric compositions having relatively small proportions of guest ions such as zinc, boron, bismuth, cerium, tungsten, copper, manganese, neodymium, niobium, silver, barium, silicon and nickel dispersed within the barium titanate crystal matrix. Such a dielectric composition can be used to form multilayer ceramic chip capacitors having internal electrodes formed of palladium or silver, or mixtures or alloys of the two.
In particular, the present invention relates to a lead-free, cadmium-free, ultra low temperature fired dielectric ceramic composition system in which the dielectric constant does not alter from its base value by more than 15 percent over a wide temperature range. Also, the dielectric constant does not alter from its base value by more than 25 percent over a wide temperature range when a DC bias voltage is applied. More specifically, this invention relates to a dielectric ceramic composition with a dielectric constant of over 2000 which is formed by firing a base ceramic oxide mixture at a temperature not exceeding 1000° C.
2. Description of Related Art
Multilayer ceramic chip capacitors (MLCC) have been widely utilized as miniature-sized, high capacitance, and high reliability electronic components. In accordance with increasing demands for high-performance electronic equipment, multilayer ceramic chip capacitors also have encountered marketplace demand for smaller size, higher capacitance, lower cost, and higher reliability. Tightening environmental regulations militate in favor of capacitors, and hence dielectric compositions that are free of lead and cadmium.
Multilayer ceramic chip capacitors generally are fabricated by forming alternating layers of a dielectric layer-forming paste and an internal electrode forming paste. Such layers are typically formed by sheeting, printing, or similar techniques, followed by concurrent firing. Generally, the internal electrodes have been formed of conductors such as palladium, gold, silver or alloys of the foregoing.
Conventional ceramic dielectric formulations lacking lead and cadmium often require sintering temperatures above 1250° C. The addition of lead or cadmium or compounds containing lead and/or cadmium, can reduce sintering temperatures from above 1250° C. to the range of about 1050° C. to about 1150° C. so that metal paste having a metal component that contains less than 80% silver and more than 20% palladium may be used as inner electrodes.
The Electronic Industry Association (EIA) prescribes a standard for the temperature coefficient of capacitance (TCC) known as the X7R characteristic. The X7R characteristic requires that the change of capacitance be no greater than ±15% over the temperature range from −55° C. to +125° C. For MLCCs designed for high voltage applications, the industry also prescribes a standard for the temperature coefficient of capacitance with a DC voltage bias applied (TCVC) known as the BX characteristic. The BX characteristic requires that the change of capacitance be no greater than ±25% over the temperature range from −55° C. to +125° C. when a DC bias voltage is applied. The percentage capacitance deviations in both the X7R and BX characteristics are measured from a reference temperature of 25° C.